If it just needs a good clean then a strong all purpose cleaner (such as Autosmart G101) and a reasonably stiff brush should shift the muck. Just make sure you keep the car really wet as you don't want to stain paintwork etc with streaks.

If totally minced, furniture clinic do a restoration kit

http://www.furnitureclinic.co.uk/Con...ration_Kit.php

I've not tried it yet (as not had any really bad roofs for a while) but all their other kit is good stuff.